1. Field of the Invention
The present invention relates to a liquid crystal device for use, for example, in a display apparatus, liquid crystal-optical shutter device and the like. More particularly, the present invention relates to a liquid crystal device which provides enhanced display performance by attaining a specific state of orientation of the liquid crystal molecules.
2. Description of the Related Arts
A liquid crystal display device has been proposed by Clark and Lagerwall in U.S. Pat. No. 4,367,924, in which light transmission is controlled by a combination of a liquid crystal element and a polarizing element by using the anisotropy of refractive index exhibited by ferroelectric liquid crystal molecules. This type of liquid crystal device is also disclosed, for example, in Japanese Laid-Open Patent Application Serial No. 56-107216. The ferroelectric liquid crystal used in this device generally exhibits non-spiral chiral smectic C (SmC*) or H (SmH*) phases within a specific temperature range. The liquid crystal has bistable characteristic: namely, it assumes either one of first and second optically stable states in response to the polarity of a sufficiently large application of an electric field and maintains such optically stable state even after electric field is discontinued. This liquid crystal also respond quickly to a change in the applied electric field. Accordingly, liquid crystals are expected to have a wide range of applications in the field of both high-speed and memory type display devices. These liquid crystals are also considered to be promising for use in large-area image displays which require a high degree of detail.
The transmittance of a liquid crystal device utilizing double refraction of a liquid crystal, under conventional conditions of crossed nicols, is expressed by the following formula: EQU I/I.sub.0 =sin.sup.2 4.theta..sub.0 sin.sup.2 (.DELTA.nd/.lambda.).pi.
where I.sub.0 represents intensity of incident light, I represents intensity of transmitted light .theta. represents tilt angle, .DELTA.n represents refractive anisotropy, d represents the thickness of liquid crystal layer and .lambda. represents wavelength of the incident light. In liquid crystal having non-spiral structure, the tilt angle .theta. is the angle of the mean molecular axis direction of the liquid crystal molecules arranged in each of the first and second stable states.
According to the formula shown above, the transmittance is maximized when the tilt angle .theta. is 22.5.degree.. From this point of view, it is desirable that the tilt angle in bistable spiral structure is as close to 22.5.degree. as is possible.
In order to obtain the desired driving characteristics for optical modulating elements incorporating this bistable liquid crystal, it is necessary that the liquid crystal interposed between a pair of parallel substrates have a molecular alignment such that the two stable states are reversibly changed effectively caused irrespective of the state of application of the electric field. Although various methods for orienting ferroelectric liquid crystal are known, the preferred orientation process is a simple rubbing process which enables the layer of smectic liquid crystal molecules to be uniaxially oriented along a normal line over a large area and which also can simplifies the production process. A suitable method for orienting ferroelectric liquid crystals, in particular, non-spiral chiral smectic liquid crystals, is proposed in U.S. Pat. No. 4,561,726 among others.
The following two problems are encountered when a known orientation method, in particular the orientation method non-spiral ferroelectric liquid crystal proposed by Clark and Lagerwall. The first problem pertains to the tilt angle of the ferroelectric liquid crystal having the non-spiral structure.
The inventors have found through experiment that the tilt angle .theta. (which will be explained in connection with FIG. 3 below) in non-spiral ferroelectric liquid crystal oriented with a conventional rubbed polyimide film is smaller than the tilt angle .THETA. (which is half the apex angle of pyramid explained in connection with FIG. 2 below) of spiral ferroelectric liquid crystal. More specifically, it was confirmed that the tilt angle .theta. in non-spiral ferroelectric liquid crystal oriented with a conventional rubbed polyimide film generally ranges between 3.degree. and 8.degree. and the transmittance is as low as 3 to 5%.
According to Clark and Lagerwall, the tilt angle &74 of a bistable non-spiral ferroelectric liquid crystal should be equal to tilt angle .THETA. of a spiral ferroelectric liquid crystal. In practice, however, the spiral angle .theta. in non-spiral structure is less than the tilt angle .THETA. in spiral structure. Moreover the fact that tilt angle .theta. is less than title angle .THETA. is attributable to the twisting arrangement of the non-spiral liquid crystal molecules. That is, non-spiral ferroelectric liquid crystal molecules are arranged in a continuous twist with respect to the line normal to the substrates, from the direction of the molecule axis of the molecules adjacent the upper substrate to the direction of the molecule axis of the molecules adjacent the lower substrate.
The second problem pertains to an afterimage which is observed in an image display using a ferroelectric liquid crystal having a non-spiral structure.
In general, an image display apparatus employs a rubbed polyimide orientation film between chiral smectic liquid crystal layer and electrodes serving as an insulating layer. When a switching voltage of one polarity is applied between the electrodes in order to switch the liquid crystal from a first optically stable state (e.g., white state to a second optically stable state (e.g., black state a reverse electric field Vrev of the reverse polarity is generated in the ferroelectric layer after the switching voltage is terminated. This reverse electric field Vrev causes an undesirable afterimage on the display. The detail of the mechanism of generation of the reverse electric field is detailed in, for example, Akio Yoshida, October, 1987, SWITCHING CHARACTERISTIC of SSFLC, pp 142-143, the subject matter of which is hereby incorporated by reference. Briefly, however, the reverse electric field is generated by switching characteristic of SSFLC.